=The self-registering gauge of past climates.=—From the initiation of the desert in its isolation from the lands tributary to the sea, its history becomes an individual and independent one. An increasing quantity of rainfall will be marked by larger inflow to the basin, and the lakes which form in its lowest depression will, as a consequence, rise and expand over larger areas. A contrary climatic change will bring about a lowering of the lakes and leave behind the marks of former shorelines above the water level (Fig. 205). Deserts are thus in a sense self-registering climatic gauges whose records go back far beyond the historic past. From them it is learned that there have been alternating periods of larger and smaller precipitation, which are referred to as _pluvial_ and _interpluvial_ periods.
[Illustration: FIG. 205.—Former shore lines on the mountain wall surrounding the desert of the Great Basin. View from the temple in Salt Lake City (after Gilbert).]
From such records it is learned that the Great Basin of the western United States was at one time occupied by two great desert lakes, the one in the eastern portion being known as Lake Bonneville (Fig. 206). With the desiccation which followed upon the series of pluvial periods, which in other latitudes resulted in great continental glaciers and has become known as the Glacial Period, this former desert lake dried up to the limits of Great Salt Lake and a few smaller isolated basins. Between 1850 and 1869 the waters of Great Salt Lake were rising, while from 1876 to 1890 their level was falling, though subject to periodic fluctuations, and in recent years the waters of the lake have risen so high as to pass all records since the occupation of the country. As a consequence the so-called Salt Lake “cut-off” of the Union Pacific Railway, constructed at great expense across a shallow portion of the lake, has been overflowed by its waters. The Sawa Lake in the Persian Desert, which disappeared some five hundred years ago, again came into existence in 1888 so as to cover the caravan route to Teheran.
FIG. 206.—Map of the former Lake Bonneville (dotted shores), and the boundaries of the Great Salt Lake of 1869 (smaller area) and that of the present (after Berghaus).]